Recently, many nanostructures having specific spatial shapes and nano-sized structures have been proposed, which are formed by self-assembly of organic or inorganic compounds through intermolecular interaction in an equilibrium or non-equilibrium state. These nanostructures can be used not only as base materials for constructing organic/inorganic composite nanomaterials having various compositions but also as templates to form nanostructures composed of various materials, and thus have attracted attention from interdisciplinary fields, industrial fields, etc.
As an example of such nanostructures, PTL 1 discloses a process of forming mesoporous silica particles that involves self-assembly of a surfactant having a specific structure in a solution, and a sol-gel reaction of a compound acting as a silica source around the self-assembled product. In another example, PTL 2 discloses a process of forming a porous membrane with a cylindrical structure having an average pore size of 1 to 200 nm that involves preparation of a phase-separated microstructure from two incompatible polymers, i.e., a water-soluble one and a water-insoluble one. Furthermore, it is well known that biopolymers, such as DNA and protein, are self-assembled into nanostructures having unique three-dimensional structures. However, few crystalline nanostructures are composed of polymers having crystallinity.
Growing layers of metal oxide around chiral nanostructures as templates to transfer the chirality of the templates to the metal oxide has also been proposed. For example, PTL 2 discloses a chiral organic/inorganic composite prepared by a reaction of a template polymer having an optically active chiral-oriented structure such as a helical structure with a metal source. The most successful example of a chiral metal oxide is a mesoporous silica having a twisted structure and synthesized using a chiral micelle template (PTL 3). Unfortunately, no crystalline chiral metal oxide has been proposed, although such amorphous chiral silica can be prepared with an organic chiral template. More recently, a chiral titanium oxide crystal has been proposed, for example, in NPTL 1. In this process, a silica source is applied onto nanocrystalline cellulose, which is a chiral organic crystal, and then calcined to prepare a chiral silica template. Titanium tetrachloride is then applied onto the surface of this hard template to grow a titanium oxide layer, and the silica template is removed with an aqueous sodium hydroxide solution.